326 INVERTEBRATE MORPHOLOGT. 



forms as Patella, for instance, having a larva destitute of some of the 

 more characteristic Veliger features and more closely resembling the 

 AnneM Trochophore and the Scaphopod larva. 



IV. Class Pelecypoda. 



The class Pelecypoda, also known as the Zamdlibranchia, 

 contains a number of fresh- water genera, though the majority 

 are marine, and all its members retain the primitive bilateral 

 symmetry of form, no visceral hump being developed. The 

 body is more or less laterally compressed and two large 

 mantle-folds (Fig. 149, m) are developed, arising one on each 

 side a short distance ventrad of the dorsal mid-line and extend- 

 ing downward so as to meet below. They thus enclose a wide 

 space, the mantle-cavity, between their inner surfaces and the 

 body-wall, within which lie the ctenidia (Fig. 148, ct) and the 

 foot (p). Upon the mantle-edge in many forms tentacles, 

 papillae, glands, and eyes are developed, and in many cases 

 the edges of the two lobes may fuse more or less completely, 

 openings being, however, left for the entrance and exit of 

 water into the mantle-cavity, and also for the protrusion of 

 the foot. All gradations of fusion are represented : thus in 

 Jfucula, Ostrea, etc., there is no fusion whatever ; in Unio 

 (Fig. 149) and other forms the posterior edges of the mantle- 

 folds are modified, so that while the edges of the folds are in 

 contact throughout the greater portion of their extent two 

 openings are left, through the uppermost of which, the exha- 

 lent opening (eo), waiter carrying with it the excreta and the 

 reproductive elements finds an exit, while through the lower 

 one, the inhalent opening {io), fresh water passes in ; in the 

 next gradation the point of separation between these two 

 openings, which in Unio was simply formed by the contact of 

 the mantle-edges, becomes permanent by the fusion of these 

 latter parts, and a further stage, seen in FereMS for example, 

 is formed by the fusion of the mantle-edges ventral to the 

 branchial opening, a fusion which may extend forward a con- 

 siderable distance. In this last condition there are three 

 openings which place the mantle-cavity in communication 

 with the exterior, one anterior, through which the foot is pro- 



